This is a hard and fast rule with all cruise lines. One passenger in the booking must remain the same from beginning to end. Despite the fact that it says that it is at the cruise line's discretion, in all my years in the business I have never seen it changed, regardless of the reason. Taking it out on Carnival might make the OP feel better but I can guarantee they would have gotten the same answer from any cruise line.
I sympathise with the OP's position, it doesn't seem fair. Having said that, there is actually a very good reason for the policy and it is not the cruise line being greedy, at least not this time.
Those of us who sell cruises are trained professionals and we are bound to certain rules and legal requirements. The agencies we work for are licenced and insured. We must make sure the person purchasing the cruise is aware of the cruise lines' terms and conditions, citizenship documentation requirements, ship, sail date, itinerary, age requirements, embarkation procedures, pre-registration and so much more. It is a fun job to be sure but one that comes with a great deal of responsibility.
If all names in a booking could be changed, it would open the door to anyone who decided, by necessity or otherwise, that they could not go n their cruse to sell it to someone else without disclosing any of the important information to the person purchasing the cruise. I know it may sound far-fetched to some but I guarantee you that if the rule were not in place, you'd see unwanted cruises on E-bay with no regulation on disclosure and that is a scary thought.
